1. Field of the Invention
This invention relates to drilling and servicing of wells. More particularly, improved apparatus and method for lifting blowout preventers or other heavy loads below the floor of a drilling rig or the main deck of a platform are provided.
2. Description of Related Art
Blowout preventers (BOPS) are used on drilling rigs to prevent continued influx of subsurface fluids into a drilling well, displacing the drilling fluid from the wellbore. As drilling fluid is displaced from the well at the surface of the earth, the hydrostatic pressure available to control influx of subsurface fluids is decreased, allowing influx of subsurface fluids at an even higher rate. If subsurface fluids flow to the surface of the earth, a “blowout” has occurred. Such occurrences are normally avoided by closing one or more of the BOPs in a BOP “stack” upon detection of significant flow of drilling fluid at the surface of a wellbore and then taking other steps to control influx of subsurface fluid.
The BOP stack is attached to the top of a casing that is cemented in the wellbore and the stack normally extends vertically to a few feet below the drilling rig floor. During the drilling process, it is often necessary to lift the BOP off the casing—to set casing slips or drilling spools, for wellhead change-outs or for other purposes. In recent years, with deeper and higher pressure wells being drilled, the weight of BOP stacks has continued to increase. At present, stacks may be used that weigh over 80 tons. Other needs may arise for lifting loads of more than 100 tons below the floor of a drilling rig or the main deck of a platform such as for salvage or recovery of equipment.
A compact winch unit placed on the drilling rig floor to lift blowout preventers is disclosed in U.S. Pat. No. 4,305,467. This patent describes the placement of two winches on a base frame, positioning the base frame on the rotary table and lifting blowout preventers by the winches. U.S. Pat. No. 5,816,565 discloses a lifting apparatus for blowout preventers in which a sliding sheave assembly is mounted to a frame assembly. The sliding sheave assembly has a first shaft with a plurality of sheaves mounted on it and a second sheave with a plurality of sheaves mounted on the second shaft. A heavy object such as a BOP stack is lifted by extending the rod end of a cylinder to increase the distance between the first and second sheaves. U.S. Pat. No. 6,053,255 discloses multiplying the safety factor of wire rope used on winch apparatus in lifting blowout preventers by employing multiple segments of cable on each of the drums of the winch units.
There is a continuing need to lift such heavy loads with equipment that can be easily mobilized and de-mobilized in the confined space of a drilling rig floor or platform and that has lifting capacity to provide a satisfactory safety factor while lifting the heaviest of BOP stacks.